Multiplexed surface acoustical wave apparatus

ABSTRACT

An apparatus is featured for controlling remote devices. The status or one or more switches associated with remote devices may be transmitted along a single transmission line. One end of the transmission line contains a surface acoustical wave (SAW) delay line. The SAW delay line responds to an interrogation pulse of radio frequency electromagnetic energy with a series of delayed pulses, each of which transmits the status of a switch by means of the presence or absence of the pulse. The SAW delay line and switches are completely passive, requiring no power source. The transmission line may optionally be interrupted and coupled by means of a pair of inductive coils. This offers the potential for rotation, as within a steering column of an automobile, without wear or damage to the transmission line. The other end of the transmission line contains electronic circuitry to provide the interrogation pulse, to decode the return delayed pulses, and to drive other circuitry as required by the switch positions.

Related Applications

This application is related to co-pending U.S. patent applications, asfollows: U.S. patent application Ser. No. 07/483,349 (now allowed),filed FEb. 20, 1990, titled CODED SURFACE ACOUSTICAL WAVE (SAW) MOTORVEHICLE SECURITY DEVICE; U.S. patent application Ser. No. 07/276,072,filed Nov. 25, 1988 (Attorney Docket No. 88-3-515), titled SPURIOUSSIGNAL CORRECTION FOR SURFACE ACOUSTIC WAVE (SAW) SECURITY DEVICES, nowU.S. Pat. No. 4,945,354; and U.S. patent application Ser. No.07/399,126, filed Aug. 28, 1989 (Attorney Docket No. 89-2-381), titledEASILY ENCODABLE SURFACE ACOUSTIC WAVE, (SAW) SECURITY DEVICES; andhereby incorporates by way of reference all of the teachings anddescription therein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus for digitally multiplexing aplurality of remote, electronically operative devices, and moreparticularly to a multiplexed apparatus utilizing a surface acousticalwave system.

In an electrical system featuring a number of diverse electronicallyoperated devices, it has been found necessary to locate the switchcontrols at a remote location. Such a system is frequently to be foundin automobiles, wherein the horn, directional signals, lights, wipers,etc., are all controllable from a wand mounted upon the steering wheelcolumn. Other automotive functions such as oil pressure, watertemperature and vehicular speed are usually mounted upon a single panelor dash.

In such cases, it may be expensive or inconvenient to provide anindividual transmission line to each device or switch. In theaforementioned motor vehicle steering wheel example, manufacturing andreliability problems are posed by passing a great number of wires up asteering column to a rotating member. In such cases, it can be desirableto employ a multiplexing technique whereby the status of more than oneswitch can be transmitted using a single transmission line. Means suchas time division multiplexing, frequency division multiplexing, andpacket transmission are well known to those skilled in the art ofelectrical communications. These well known multiplexing methods requirethat sophisticated electronic circuits requiring power be installed atboth ends of the transmission line.

It would be desirable to replace the electronic circuitry at one end ofthe transmission line with a passive component not requiring power.Using the steering wheel and tire pressure sensor switch as examples, itwould also be desirable to have a simple and reliable means to transmitsuch data using a noncontacting connector with freedom to rotate.

The present invention has developed a system that can control and/orinterrogate remote, electronically operative devices. A singletransmission line can carry multiplexed digital information to a pair ofcoupling inductive coils. One of the coils is associated with aprogrammable surface acoustical wave device.

A surface acoustical wave transponder that responds to an interrogatingpulse with a delayed sequence of pulses, and a circuit for comparing theproper pulse sequence is described in the, aforementioned U.S. patentapplication Ser. No. 07/483,349. The circuit described therein (see FIG.8) can be utilized in the present invention to differentiate betweenswitching signals for the remote devices, which switching signals arepropagated as reflected surface acoustical waves from the programmabletransducers of the SAW device located within the steering wheel column.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate the disadvantages ofthe prior art.

It is another object of this invention to provide a multiplexed SAWapparatus for interrogating and/or controlling a number of remotedevices.

It is another object of the invention to provide a passive SAW systemfor controlling or interrogating a number of remote devices.

It is a further object of this invention to control or interrogateremote devices without direct electrical

e connection, utilizing a coil coupled SAW device.

These objects are accomplished, in one aspect of the invention, by theprovision of a multiplexed apparatus for controlling and/orinterrogating a number of remote devices. A SAW transponder contains aninterface transducer and a plurality of programmable SAW reflectorsdisposed upon a piezoelectric substrate.

An interrogating signal is imparted to the SAW transducer either bydirect connection to a signal source by means of a cable or by means ofinductive couping coils. The electrical interrogating signal isconverted to SAW energy by means of the SAW transducer and travels alongthe surface of a peizoelectric substrate such as lithium niobate. Uponencountering a programmable reflector, the SAW energy either passesunimpeded or a portion of its energy is redirected back towards the SAWtransducer, depending on which function has been programmed into thereflector.

The programming of the transducer is accomplished by setting anelectrical switch to the open or closed position. The reflected SAWenergy reaches the SAW transducer after a delay time and is reconvertedto electrical energy which can then be interpreted by electroniccircuitry. The individual reflectors are distinguished by the uniquetime of the transponded pulse produced by each reflector.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and other objects of this invention will become moreapparent and will be better understood with reference to the followingdetailed description considered in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a plan view of transponder constructed in accordance with theprinciples of this invention;

FIG. 2 is a plan view of a transponder constructed in accordance withthe invention including a plurality of switchable transducers;

FIG. 3 is a photographic view of an electrical response from the SAWdevice illustrated in FIG. 2 with all the switchable transducers in anon-reflective state; and

FIG. 4 is a photographic view of an electrical response from the SAWdevice illustrated in FIG. 2 with four switchable transducers in areflecting state.

BEST MODE FOR CARRYING OUT THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawings.

Generally speaking, the invention relates to a multiplexed apparatus forinterrogating the status of a number of switches and/or a number ofremote devices. The apparatus features a SAW device having a number ofprogrammable or switchable transducers. By means of opening or closing arespective switch associated with each one of the programmabletransducers, the transducers are caused to change their acoustical wavestate between a first normally non-reflective surface acoustical wavestate, and a second reflective surface acoustical wave state.

Remote devices respectively associated with each transducer can thus becontrolled and/or interrogated by programming the transducers to be ineither one of the two operative states.

For the purpose of brevity like elements will have the same numericaldesignation throughout the figures.

Now referring to FIG. 1 an example of a SAW transponder 100 is showncomprising a piezoelectric substrate 110, in accordance with the presentinvention. In the figure, transducer 130 disposed on substrate 110 isconnected to an inductive coil 230, preferably designed so that itsinductance resonates with the capacitance of the transducer inaccordance with well known principles. A so-called "split electrode"transducer design is shown in FIG. 1, with electrodes spaced at quarteracoustic-wavelengths of the fundamental SAW frequency. It is alsopossible to employ a so-called "single electrode" transducer in whicheach split electrode is replaced with a single unsplit electrode spacedat half acoustic-wavelengths of the fundamental SAW frequency. Whenexcited by an interrogation pulse, transducer 130 launches SAW 210towards the switchable reflector 160.

The switchable reflector 160 disposed on substrate 110 in FIG. 1 is alsoa "split electrode" transducer. It may or may not have the same numberof electrodes as transducer 130. The switchable reflector 160 has aproperty by which it is able to transmit SAW's without reflection, whenit is shorted by a closed switch 240. However, if the switch is opened,the switchable reflector becomes partially reflecting, creating areflected SAW 220 which can be transduced to electromagnetic energy andretransmitted by transducer 130 to coil 230. The switchable reflectormay also be constructed from a "single electrode" transducer, but therole of the switch may be reversed; that is, shorting the switch 240 maylead to reflection and vice versa. The "split electrode" configurationis preferred for the switchable reflector.

Another embodiment of the present invention is illustrated in FIG. 2.Here is shown a transponder 120 with one transducer 180 and sixswitchable reflectors 170, 171, 172, 173, 174 and 175. Each switchablereflector is connected to a switch, not shown, by a twisted pair ofplastic insulated #22 wire of length 7.5 cm (Experimentation has shownthat twisted pairs up to 15 cm length may be used without degradation ofsignals.) The substrate 11 is Y-cut Z-propagating lithium niobate.Lithium niobate was chosen for its high coupling coefficient; the YZ cutwas chosen for its ruggedness. The electrode spacings were chosen sothat the fundamental frequency of the transducers was 44.3 MHz. The useof low frequencies below 100 MHz and preferably below 50 MHz ispreferred since low cost electronic circuitry and switches can be used.Moreover, simple photolithographic techniques can be used to manufacturethe SAW transponders (typical linewidths=10 microns).

Sample voltages from the pickup coil, not shown, coupled to coil 230attached to transducer 180 in FIG. 2 are shown in FIGS. 3 and 4. In FIG.3, all switches are closed. In FIG. 4, four switches (connected toswitchable reflectors 170, 171, 173 and 175) are open, yielding fourreadily detectable pulses. These pulses can be detected, and decoded byelectronic circuitry 310 shown in FIG. 8 of, and described in U.S.patent application, Ser. No. 07/483,349.

In the preferred embodiment electrodes of the transducers are spaced atquarter acoustic-wavelengths, so that reflection from individualelectrodes is subject to destructive interference and is completelycancelled. When the reflector transducer is open circuited, the incominginterrogating SAW causes buildup of voltage across the transducer, whichin turn causes re-radiation of SAW's in both directions; there-radiation towards the interrogating transducer constitutes partialreflection. This phenomenon is known as regeneration. When thereflector-transducer is shorted, this voltage cannot build up and noreflection occurs.

The switches, not shown, which respectively open or close the oppositeline ends of each transducer 170 through 175 are associated with theremote devices, and thus provide the means to control or interrogatethese devices. These switches modulate the reflective nature of thetransducers which can be sensed by the electronic circuit.

Since other modifications and changes varied to fit particular operatingrequirements and environments will be apparent to those skilled in theart, the invention is not considered limited to the example chosen forpurposes of disclosure, and covers all changes and modifications whichdo not constitute departures from the true spirit and scope of thisinvention.

Having thus described the present invention, what is desired to beprotected by Letters Patent is presented by the following appendedclaims.

What is claimed is:
 1. An interrogation device supported in anautomotive vehicle and operatively connected by means of a coil to asurface acoustical wave apparatus for interrogating the operative statusof remote electronically operative automobile devices that areelectrically isolated from said interrogation device by reason of saidcoil, comprising:a medium having means defining a surface acousticalwave path; an interface transducer carried by said medium for convertingelectromagnetic energy to surface acoustical waves, and for receivingreflected surface acoustical waves and converting said reflected surfaceacoustical waves to electromagnetic energy; interrogation meansoperatively connected to said interface transducer through a coil, forreceiving electromagnetic energy from said interface transducer as afunction of the operative status of electrically isolated,electronically operative automotive devices; and a plurality ofprogrammable, spaced-apart, normally non-reflective transducers disposedalong said surface acoustical wave path and positioned to receivesurface acoustical waves from said interface transducer, saidnon-reflective transducers each operatively connected to andrespectively programmable by a remotely located automotive device tobecome reflective and to reflect surface acoustical waves back to saidinterface transducer for converting the reflected waves toelectromagnetic energy as a function of an operative state of each ofsaid automotive devices.
 2. The interrogation device of claim 1, whereineach programmable transducer is caused to become reflective of surfaceacoustical waves by means of a respective switch, each of saidprogrammable transducers having opposite line ends that are normallyelectrically coupled together by respective ones of said switches toprovide a non-reflective state.
 3. The interrogation device of claim 1,wherein each of said programmable transducers has a plurality of splitlines arranged transversely to said surface acoustical wave path.
 4. Theinterrogation device of claim 1, comprising a time domain reflectometrycircuit.
 5. The interrogation device of claim 1, including a circuitthat can differentiate between differences in electromagnetic energiesproduced by respective, reflected acoustical waves.
 6. The interrogationdevice of claim 1, wherein each of said automotive devices ismultiplexed.
 7. The interrogation device and surface acoustical waveapparatus of claim 1, wherein digital information representative of theoperative status of remote automotive devices is transmitted through asingle transmission line disposed along a steering column, said singletransmission line coupled to one of a pair of inductive coilsoperatively connected to said interrogation device.
 8. The interrogationdevice of claim 1, wherein said medium comprises a piezoelectricsubstrate.
 9. The interrogation device of claim 8, wherein saidpiezoelectric substrate comprises lithium niobate.
 10. A multiplexed,interrogation device and surface acoustical wave apparatus combinationfor interrogating a number of switches coupled to a plurality ofswitchable transducers supported by said surface acoustical waveapparatus to determine the operational status of remote, electronicallyoperative devices that are electrically isolated from said interrogationdevice, comprising:a surface acoustical wave medium having a surfaceacoustical wave path; an interface transducer disposed along saidsurface acoustical wave path for converting electromagnetic energy tosurface acoustical waves and reflected surface acoustical waves backinto electromagnetic energy; an interrogation device operatively coupledto said interface transducer disposed along said surface acoustical wavepath; and a plurality of switchable transducers disposed along saidsurface acoustical wave path spaced apart from said interfacetransducer, said switchable transducers each being respectivelyswitchable by a corresponding switch, and operative between twooperative states, a first state being normally non-reflective to surfaceacoustical waves, and a second state being reflective thereto, each ofsaid two states being a function of a different operative state of arespective, remote, electronically operative device connected to eachrespective switchable transducer through said corresponding switch. 11.The multiplexed, surface acoustical wave apparatus and interrogationdevice combination of claim 10, wherein said interrogation devicecomprises a time domain reflectometry circuit.
 12. The multiplexed,surface acoustical wave apparatus and interrogation device combinationof claim 11, wherein said interrogation device comprises a circuit thatcan differentiate between differences in electromagnetic energiesproduced by respective, reflected acoustical waves.
 13. The multiplexed,interrogation device and surface acoustical wave apparatus combinationof claim 10, wherein said surface acoustical wave medium comprises apiezoelectric substrate.
 14. The multiplexed, interrogation device andsurface acoustical wave apparatus combination of claim 13, wherein saidpiezoelectric substrate comprises lithium niobate.
 15. The multiplexed,interrogation device and surface acoustical wave apparatus combinationof claim 10, wherein each switchable transducer comprises normallyclosed line ends that are opened by a respective switch to provide saidrespective switchable transducer reflections of surface acousticalwaves.
 16. The multiplexed, interrogation device and surface acousticalwave apparatus combination of claim 10 for use in an automotiveelectrical system.
 17. A method of controlling or interrogating a numberof remote, electrically isolated devices in an electrical system bymeans of a surface acoustical wave apparatus having a coded sequence ofprogrammable transducers, each of which is respectively associated witha remote device in said electrical system, said method comprising thesteps of:(a) programming select ones of said programmable transducers toprovide a means to control or interrogate certain ones of said remotedevices; and (b) multiplexing said surface acoustical wave apparatus,whereby each remote device can be controlled or interrogated over asingle cable.
 18. The method of claim 17, wherein said electrical systemis disposed within an automobile and said surface acoustical waveapparatus is carried upon a steering wheel column thereof.
 19. Themethod of claim 17, further comprising the step of:(c) programming saidselect ones of said programmable transducers to be operative in one oftwo states, a first, wave reflective state, and a second, wavenon-reflective state.
 20. The method of claim 19, wherein saidprogrammable transducers are normally in said second, wavenon-reflective state.